Ruifa Zong

Preparation and study of a family of dinuclear Ru(II) complexes that catalyze the decomposition of water.

An approach is developed for the four-electron oxidation of water to provide dioxygen that involves the juxtaposition of two Ru(II) centers such that a metal-bound water molecule might interact with one or both of the metals. The key element in this approach is an appropriate bridging ligand that will hold the metal assembly intact through the full redox cycle. Various synthetic approaches to such ligands are described with the ultimate preparation of four closely related bis-tridentate polypyridine-type systems in which the bridging and distal portions of the ligand are varied. All of these ligands self-assemble with two Ru(II) centers bridged by a Cl ion in the equatorial plane and four axial monodentate substituted pyridines or N-methylimidazoles to form the well-organized catalyst complexes. These complexes are characterized by their distinctive (1)H NMR spectra as well as an X-ray structure of one representative species. The photophysical and electrochemical features of these complexes are consistent with electronegativity and delocalization effects in the equatorial and axial ligands. Of the 14 complexes studied, all but 2, which each contain four axial N-methylimidazole ligands, catalyze the decomposition of water in the presence of excess Ce(IV) as a sacrificial oxidant at pH = 1. Both the rates of oxygen evolution and the catalyst turnover numbers (TNs) are measured. For the active catalysts, the relative rates vary from 1 to 51 and the TNs measure from 80 to 689. Various analytical methods for making these measurements are discussed, and it is found that there is an approximately linear relationship between the rate and TN. Future work will involve optimization of these systems and studies aimed at a better understanding of the mechanism.

Exploitation of Long-Lived 3IL Excited States for Metal–Organic Photodynamic Therapy: Verification in a Metastatic Melanoma Model

Members of a family of Ru(II)-appended pyrenylethynylene dyads were synthesized, characterized according to their photophysical and photobiological properties, and evaluated for their collective potential as photosensitizers for metal-organic photodynamic therapy. The dyads in this series possess lowest-lying (3)IL-based excited states with lifetimes that can be tuned from 22 to 270 μs in fluid solution and from 44 to 3440 μs in glass at 77 K. To our knowledge, these excited-state lifetimes are the longest reported for Ru(II)-based dyads containing only one organic chromophore and lacking terminal diimine groups. These excited states proved to be extremely sensitive to trace amounts of oxygen, owing to their long lifetimes and very low radiative rates. Herein, we demonstrate that (3)IL states of this nature are potent photodynamic agents, exhibiting the largest photocytotoxicity indices reported to date with nanomolar light cytotoxicities at very short drug-to-light intervals. Importantly, these new agents are robust enough to maintain submicromolar PDT in pigmented metastatic melanoma cells, where the presence of melanin in combination with low oxygen tension is known to compromise PDT. This activity underscores the potential of metal-organic PDT as an alternate treatment strategy for challenging environments such as malignant melanoma.

Effects of a proximal base on water oxidation and proton reduction catalyzed by geometric isomers of [Ru(tpy)(pynap)(OH2)]2+.

Basic difference: The importance of a pendent base in promoting proton-coupled electron-transfer reactions with low activation barriers has been discussed for H(+) reduction or H(2) oxidation in acetonitrile. Investigation of the interaction between a base positioned in the second coordination sphere of a complex and a water ligand in water oxidation reactions using geometric isomers of [Ru(tpy)(pynap)(OH(2))](2+) (see picture) gave intriguing results.

Photobiological Activity of Ru(II) Dyads Based on (Pyren-1-yl)ethynyl Derivatives of 1,10-Phenanthroline

Several mononuclear Ru(II) dyads possessing 1,10-phenanthroline-appended pyrenylethynylene ligands were synthesized, characterized, and evaluated for their potential in photobiological applications such as photodynamic therapy (PDT). These complexes interact with DNA via intercalation and photocleave DNA in vitro at submicromolar concentrations when irradiated with visible light (lambda(irr) > or = 400 nm). Such properties are remarkably sensitive to the position of the ethynylpyrenyl substituent on the 1,10-phenanthroline ring, with 3-substitution showing the strongest binding under all conditions and causing the most deleterious DNA damage. Both dyads photocleave DNA under hypoxic conditions, and this photoactivity translates well to cytotoxicity and photocytotoxicity models using human leukemia cells, where the 5- and 3-substituted dyads show photocytotoxicity at 5-10 microM and 10-20 microM, respectively, with minimal, or essentially no, dark toxicity at these concentrations. This lack of dark cytotoxicity at concentrations where significant photoactivity is observed emphasizes that agents with strong intercalating units, previously thought to be too toxic for phototherapeutic applications, should not be excluded from the arsenal of potential photochemotherapeutic agents under investigation.

Direct access to 4-carboxy-1,8-naphthyridines and related compounds through Pfitzinger-type chemistry.

The 4-carboxy-1,8-naphthyrid-2-yl moiety is a useful ligand component in that it promotes lower energy electronic absorption in its metal complexes and also provides a useful tether for anchoring the ligand to a semiconductor surface. The synthon [2-(pivaloylamino)pyrid-3-yl]oxoacetic acid ethyl ester can be easily obtained in two steps from 2-aminopyridine. The Pfitzinger-type condensation of this molecule with a 2-acetylazaaromatic species in ethanolic KOH, after acidification, directly provides bi- and tridentate ligands containing the 4-carboxy-1,8-naphthyrid-2-yl moiety.

Ultrafast structural dynamics of Cu(I)-bicinchoninic acid and their implications for solar energy applications.

In this study, ultrafast optical transient absorption and X-ray transient absorption (XTA) spectroscopy are used to probe the excited-state dynamics and structural evolution of copper(I) bicinchoninic acid ([Cu(I)(BCA)2](+)), which has similar but less frequently studied biquinoline-based ligands compared to phenanthroline-based complexes. The optical transient absorption measurements performed on the complex in a series of polar protic solvents demonstrate a strong solvent dependency for the excited lifetime, which ranges from approximately 40 ps in water to over 300 ps in 2-methoxyethanol. The XTA experiments showed a reduction of the prominent 1s → 4pz edge peak in the excited-state X-ray absorption near-edge structure (XANES) spectrum, which is indicative of an interaction with a fifth ligand, most likely the solvent. Analysis of the extended X-ray absorption fine structure (EXAFS) spectrum shows a shortening of the metal-ligand bond in the excited state and an increase in the coordination number for the Cu(II) metal center. A flattened structure is supported by DFT calculations that show that the system relaxes into a flattened geometry with a lowest-energy triplet state that has a dipole-forbidden transition to the ground state. While the short excited-state lifetime relative to previously studied Cu(I) diimine complexes could be attributed to this dark triplet state, the strong solvent dependency and the reduction of the 1s → 4pz peak in the XTA data suggest that solvent interaction could also play a role. This detailed study of the dynamics in different solvents provides guidance for modulating excited-state pathways and lifetimes through structural factors such as solvent accessibility to fulfill the excited-state property requirements for efficient light harvesting and electron injection.